I'm having a mental block at the moment.  This seriously can't be hard...

I'm attempting to render an image_map of a previously traced scene - the
goal of which is to allow me the luxury of rendering a frame/border/special
effect of some sort around the original image.

I need to re-render my image_map such that every pixel traced is correlated
to the original image_map pixel.  How can I set up my image_map (distance
from camera, mainly) to ensure this to be the case?  After some basic math,
I've gotten close, but it's not a perfect match.

Any thoughts?

- How

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"How Camp" <hac### [at] gmailcom> wrote in message 
news:web.434eaaf55d09ff87af56bf270@news.povray.org...
> I'm having a mental block at the moment.  This seriously can't be hard...
>
> I'm attempting to render an image_map of a previously traced scene - the
> goal of which is to allow me the luxury of rendering a 
> frame/border/special
> effect of some sort around the original image.
>
> I need to re-render my image_map such that every pixel traced is 
> correlated
> to the original image_map pixel.  How can I set up my image_map (distance
> from camera, mainly) to ensure this to be the case?  After some basic 
> math,
> I've gotten close, but it's not a perfect match.
>
> Any thoughts?
>
> - How
>

If you're using a perspective camera you'll get problems because you'll need 
to adjust for the distortion that perspective introduces (and that is likely 
to be inordinately difficult). You should be able to achieve this fairly 
simply with an orthographic camera.

Chris B.

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Chris B <c_b### [at] btconnectcomnospam> wrote:
> If you're using a perspective camera you'll get problems because you'll need 
> to adjust for the distortion that perspective introduces (and that is likely 
> to be inordinately difficult).

  If the plane containing the image map is completely perpendicular to
the direction of the camera, can you explain what kind of perspective
distortion happens there?

-- 
                                                          - Warp

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"Warp" <war### [at] tagpovrayorg> wrote in message 
news:434eccf0@news.povray.org...
> Chris B <c_b### [at] btconnectcomnospam> wrote:
>> If you're using a perspective camera you'll get problems because you'll 
>> need
>> to adjust for the distortion that perspective introduces (and that is 
>> likely
>> to be inordinately difficult).
>
>  If the plane containing the image map is completely perpendicular to
> the direction of the camera, can you explain what kind of perspective
> distortion happens there?
>
> -- 
>                                                          - Warp

Ah. I think I'm wrong. I was thinking of the distortion you get when a flat 
surface is projected onto the curved surface of a retina, but of course the 
camera in a ray tracer is a perfect point and the projection is onto a flat 
plane, so you should be able to get a mapping from one plane to another. 
With the default image_map scaling giving you an image from <0,0,0> to 
<1,1,0> you should then be able to set the up vector to y and the right 
vector to x with the camera at <0.5,0.5,-1> and look_at <0.5,0.5,0> and get 
a perfect pixel for pixel mapping.

Chris B.

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> With the default image_map scaling giving you an image from <0,0,0> to
> <1,1,0> you should then be able to set the up vector to y and the right
> vector to x with the camera at <0.5,0.5,-1> and look_at <0.5,0.5,0> and get
> a perfect pixel for pixel mapping.

Well, I'm silly.  Yes, this is what I was doing... but I applied an
antialias value without thinking about it.  Blush.

Thanks, both Chris and Warp.

- How

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Hmph.  Okay, I'm still a bit of a dunce.

My image now renders perfectly in terms of pixel reconstruction, but the
colors of the 'rerendered' image are significantly darker than the original
copy.

I have no light sources, and my ambient value is 1.  What else am I
forgetting?

- How

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How Camp <hac### [at] gmailcom> wrote:
> My image now renders perfectly in terms of pixel reconstruction, but the
> colors of the 'rerendered' image are significantly darker than the original
> copy.

  Probably gamma issues.
  Do you have a assumed_gamma setting in your scene? If yes, comment it out.
If no, then try something like 1.0 or 2.2. Or something.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:

>   Probably gamma issues.
>   Do you have a assumed_gamma setting in your scene? If yes, comment it out.
> If no, then try something like 1.0 or 2.2. Or something.

You're good, Warp.  :)

No, I don't have assumed_gamma.  I added it, with a value of 1.0, and things
are better, but not perfect.  I ended up lowering the image_map's ambient
value to some screwy value (0.57) to compensate.  Not sure why this is.

My original image has no specific assumed_gamma value, either.  Naively, I
would think if I have identical assumed_gamma values in my original and my
're-rendered' scene, shouldn't I get the same color from each pixel?

Were I to add an assumed_gamma of 1.0 to my original image, I would not
expect any improvement, since originally both renders lacked any
assumed_gamma setting.

- How

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How Camp wrote:
> No, I don't have assumed_gamma.  I added it, with a value of 1.0,
> and things are better, but not perfect.  I ended up lowering the
> image_map's ambient value to some screwy value (0.57) to
> compensate.  Not sure why this is.

	I would go with an assumed gamma of 1/2.2 or 1/2.5 (whichever your
disply gamma is) along with the ambient 1.0

		Jerome
- --
******************************
*      Jerome M. Berger      *
*  mailto:jeb### [at] freefr   *
*  http://jeberger.free.fr/  *
******************************
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I'm sorry.. but perhaps I don't understand your problem entirely.
How about smacking that old render onto a plane, scale it down,
add the border and render with a orthographic camera?

Stefan

"How Camp" <hac### [at] gmailcom> skrev i meddelandet 
news:web.434eaaf55d09ff87af56bf270@news.povray.org...
> I'm having a mental block at the moment.  This seriously can't be hard...
>
> I'm attempting to render an image_map of a previously traced scene - the
> goal of which is to allow me the luxury of rendering a 
> frame/border/special
> effect of some sort around the original image.
>
> I need to re-render my image_map such that every pixel traced is 
> correlated
> to the original image_map pixel.  How can I set up my image_map (distance
> from camera, mainly) to ensure this to be the case?  After some basic 
> math,
> I've gotten close, but it's not a perfect match.
>
> Any thoughts?
>
> - How
>
>
>

Post a reply to this message